Especially in technical drawings for turned parts, the tolerance mark for the roundness of a part will be found.
This shape tolerance is important when describing cylinders and tapers.
The toleranced circumferential line must lie within two concentric circles with specified spacing in all cutting planes that are executed perpendicular to the center axis.
The tolerance zone is therefore perpendicular to the axis.

Circularity  in Injection Molding

Quality Assurance: Tolerance zone by two concentric circles at a distance of 0.03mm perpendicular to the center line

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.